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3/4/2019
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SPH 302
THERMODYNAMICS
Nyongesa F. W., PhD.
e-mail: [email protected]
UNIVERSITY OF NAIROBI
Objectives
Explain the Laws of thermodynamics & their significance
Apply laws of thermodynamics to solve problems relating to energy conversion processes
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UNIVERSITY OF NAIROBI 3/4/2019 2:35 PM SPH 302
Nyongesa F. W., Ph.D 3
Course Outline
Lecture 1: Thermodynamic concepts & Zeroth Law
Lecture 2: 1ST Law of Thermodynamics
Lecture 3: 2ND Law
Lecture 4: Entropy and 2ND Law
Lecture 5: Phase Changes & Equilibria
UNIVERSITY OF NAIROBI 3/4/2019 2:35 PM SPH 302
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Prerequisites
SPH 203
Calculus & ODE
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INTRODUCTION
UNIVERSITY OF NAIROBI
Before industrial revolution, “machinery” were powered by animals
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UNIVERSITY OF NAIROBI
UNIVERSITY OF NAIROBI
James watt Steam Engine
1ST Engine (invented 1790) to
convert steam (Heat) to mechanical work
Lead to industrial revolution
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UNIVERSITY OF NAIROBI
Steam Engines of Industrial Revolution
UNIVERSITY OF NAIROBI
Steam powered cars
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UNIVERSITY OF NAIROBI
Steam powered bicycles
UNIVERSITY OF NAIROBI
Steam powered Industries
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Work output
Steam (Heat)
By 19th cent, a new science was born – THERMODYNAMICS
GOAL: Study & improve efficiency of devices that convert heat work i.e., heat engines
UNIVERSITY OF NAIROBI
1930s - 1ST Diesel Engines (internal combustion) developed
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UNIVERSITY OF NAIROBI
2000 - Hybrids & gas turbines
UNIVERSITY OF NAIROBI 3/4/2019 2:35 PM SPH 302
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What is Thermodynamics?
Thermo Energy transfer in form of heat
Dynamics Motion in form of mechanical work
Thermodynamics = science that govern energy conversions processes (heat work and vice versa)
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UNIVERSITY OF NAIROBI 3/4/2019 2:35 PM SPH 302
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Why Thermodynamics ?
It provides the Laws of nature that govern energy conversion processes
ENERGY
Chemical Thermal
Solar Mechanical
Energy 1 (heat)
Energy 2 (work)
Laws of Thermodynamics
Relationship?
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Food (Energy) input
Waste
Work out
Examples – Digestive system
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UNIVERSITY OF NAIROBI 3/4/2019 2:35 PM SPH 302
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Mechanical devices
W
waste
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Heat pumps
Work in
Heat out
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Laws of Thermodynamics
Energy conversions processes are governed by 4 Laws (of Nature)
Zeroth Law Gives condition for heat
exchange between bodies in contact
UNIVERSITY OF NAIROBI 3/4/2019 2:35 PM SPH 302
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1ST Law
Gives relationship between Heat input (dQ) into system, internal energy (dU) and work output (dW)
dQ = dU + dW
dQ dU
dW
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“No animal works continously without eating or wearing out”
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Second Law:
(i) Defines efficiency of processes that convert heat work. No heat engine or animal has 100% efficiency.
“If you eat, you must shit”.
Fuel Tank Engine
No exhaust
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Food input)
Impossible
Work out
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Systems become Disorderly with time
(ii) Gives direction of natural occurring processes - “Arrow of time”
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d (disorder)/dt) > 0
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Third Law:
Explains behaviour of systems as T absolute zero (0K) i.e., systems become orderly
E.g., water ice
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Lecture 1
Thermodynamic Concepts & Zeroth Law
Ideal Gas Laws Zeroth Law
UNIVERSITY OF NAIROBI 3/4/2019 2:35 PM SPH 302
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Objectives
Explain thermodynamic
concepts
Explain ideal gas Laws
Explain Zeroth Law and its
significance
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UNIVERSITY OF NAIROBI 3/4/2019 2:35 PM SPH 302
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Thermodynamic concepts
Energy conversion processes are studied using a thermodynamic system
System – its Part of material universe that can be isolated completely from the rest for investigation
SYSTEM + SURROUNDING = UNIVERSE
system sorrounding boundary
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Surrounding - The rest of the universe in the neighbourhood of the system.
To a thermodynamic system two ‘things’ may be added/removed: energy (in the form of heat &/or work)
matter
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Types of systems
3 types
(a) Open systems:- Allows exchange of both heat and matter through the boundary. E.g. digestive system.
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(b) Closed systems:- Allows only exchange of heat with surrounding but not matter. E.g. refrigerator
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(c) Isolated System:- No heat or matter exchange occurs with surrounding. The walls are Adiabatic (Adiathermal) E.g. Vacuum flask.
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Thermodynamic Variables:- Are parameters that describe behavior or state of system i.e., P, T, V & composition ()
Extensive variable:- Dependent on mass/size of the substance present in the system e.g., internal energy U.
Intensive variable:- independent on size of system
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Heat:- Transfer of thermal energy between systems by virtue of a temp difference
Work:-Transfer of mechanical energy
Both Heat & Work = ways of
transferring energy
“Bodies contain internal energy (U) and not heat”
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Thermodynamic equilibrium:- State where system experiences thermal, mechanical and chemical equilibrium S.T. P, V, T & are const in time.
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Working substance:- Fluid enclosed in the system that either receives or transfers energy to the surrounding in the form of heat or work.
In Thermodynamics, we use the ideal gas as the working substance. WHY?
UNIVERSITY OF NAIROBI 3/4/2019 2:35 PM SPH 302
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Activity
Name the working substance in these systems
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Lecture Evaluation
Explain the following
Open & Closed system
Adiabatic & Diathermal Wall
Extensive & Intensive variables
Heat and work
Thermodynamic Equilibrium
Behavior of Ideal gases
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An ideal gas is an abstraction (i.e., theoretical gas) that obeys Ideal gas laws (Eqn of state).
Assumptions
(i) It is composed of randomly moving point particles whose only interactions are perfectly elastic collisions.
(ii) The particles occupy negligible volume compared to the bulk volume of the gas
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In Thermodynamics, we use the concept of the ideal gas because:
It satisfactorily models behaviour of real gases under classical mechanics
it obeys simple gas laws.
Real gases (N2, O2, H2 etc and fluids) fail to obey the ideal gas model fully because:
gas molecules occupy finite volume and liquefy at low T and at high P
Intermolecular attractions and collisions are not elastic
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UNIVERSITY OF NAIROBI
Consider an ideal gas enclosed in a container with movable piston
Behavior of ideal gas depends on T, P and V and obey the gas laws:
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Ideal Gas Laws
UNIVERSITY OF NAIROBI 3/4/2019 2:35 PM SPH 302
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VP
1
V
P
PV = c CPV
(a) Boyle’s law:
For const T, Compressing a gas, increases P
P, V
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(b) Pressure law: For const Vol, Heating a gas
increases P
(c) Charles’ Law: For Const P, Heating a gas
increases V
CT
PTP
CT
VTV
T
P
T
V
-273 K
-273 K
P, V
UNIVERSITY OF NAIROBI 3/4/2019 2:35 PM SPH 302
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(d) Avogadros Law: V No. of moles (n) of the gas i.e.
V n
Combining (a) to (d) gives
PV = nRT ……………………..(1.1)
Eqn (1) defines the State of a system
Equation of state
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UNIVERSITY OF NAIROBI 3/4/2019 2:35 PM SPH 302
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P-V Diagram
From PV = nRT Behaviour/processes of ideal gas can be represented on PV diagrams
V
P
PV = c
UNIVERSITY OF NAIROBI 3/4/2019 2:35 PM SPH 302
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Real Gases
Real gases (N2, O2, H2 etc and fluids) behave like ideal gas only at high temps and low pressures where there is less intermolecular attractions.
At low temps or higher pressures, real gases condense to liquid (due to increases intermolecular forces) and fail to obey the ideal gas model.
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UNIVERSITY OF NAIROBI 3/4/2019 2:35 PM SPH 302
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Eqn of state of real gas is given by
Where a & b molecular attractions & v = molecular volume
nRTnbVv
aP
2
UNIVERSITY OF NAIROBI 3/4/2019 2:35 PM SPH 302
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Activity
State an ideal gas
Explain why we use an ideal gas in the study of thermodynamics
Explain conditions under which real gases approach the ideal gas behaviour
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UNIVERSITY OF NAIROBI 3/4/2019 2:35 PM SPH 302
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Zeroth law of Thermodynamics
Naturally, a hot object looses heat to attain thermal equilibrium with surrounding
Zeroth Law Gives condition for heat exchange between bodies n contact
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“Statement of Zeroth Law”
"If objects A and B are separately in thermal equilibrium with third object C, then A and B are in thermal equilibrium with each other”.
A C
B
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Significance of Zeroth Law
Defines condition for thermal equilibrium between bodies
Introduces concept of temperature and how it is determined – Through thermal equilibrium
Temperature = measure of degree of hotness
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Lecture -Evaluation
1. State Equation of State
2. State Zeroth Law and its significance